As magnetic recording is pushed to greater storage densities through higher linear densities and narrower track widths, magnetoresistive (MR) reproduce heads are increasingly recognized as the technology of choice. Two technological hurdles presently stand in the way of practical application of MR heads: the establishment of an optimum bias magnetization distribution for linearization of the reproduce signal; and stabilization of domain-wall-free, quasi-single domain magnetization throughout the central (active) region of the MR element. Permanent magnet (PM) biasing methods are particularly attractive in that they require no power dissipation, nor entail any loss of signal due to shunting of the MR sense current.
U.S. Pat. No. 4,903,158 issued Feb. 20, 1990 to Smith, teaches the use of a deposited PM element having the same or nearly the same geometry as the MR element and overlying the MR element. As illustrated in FIG. 2, the MR head, generally designated as 10, includes: (1) a PM element 12 deposited on a substrate (not shown) and magnetized in the direction of arrow A; and an MR element 14 deposited over the PM element on the substrate. In the descriptions of the drawings that follow, the MR head will always be designated 10, the PM element will be designated as 12 and the MR element will be designated 14. The deposited PM element 12 provides a very spatially non-uniform longitudinal stabilization field which complementarily cancels the similarly non-uniform internal demagnetization field of the MR element 14 (which is the fundamental source of single domain instability). The underlying deposited PM element does not apply excessive longitudinal field to the central active area of the MR element 14, which would reduce the reproduce sensitivity of the MR head. A problem with the MR head taught in the '158 patent is that the relatively large magnetic field at the edge of the PM element 12 adjacent the head medium interface can distort the information recorded on the magnetic medium. Yet another problem with the MR head arises because the inhomogeneities in the closely adjacent PM can perturb the magnetic and/or the reproduce properties of the MR element. In addition, the PM head positions the PM adjacent to the planar surface of the MR head. When the PM is in this position, local stray fields from inhomogeneities in the PM film have a degrading effect on the magnetic configuration of the MR element.
U.S. Pat. No. 4,972,284 issued Nov. 20, 1990 to Smith et al teaches an alternative configuration for a deposited PM element which simultaneously provides both a transverse field for establishing a bias magnetization in the MR element, and a unidirectional longitudinal field component for maintaining a domain-wall-free magnetization state in the active region of the MR element. According to the teaching of the '284 patent, the deposited PM element is in the shape of a "C" or "L" and the PM element is coplanar with the MR element. As shown in FIG. 3, the C-shaped PM element 12 is located adjacent and in the same plane as the MR element 14. The PM element 12 is magnetized at an angle .theta. with respect to its longitudinal axis, thereby providing both a transverse field for establishing a bias magnetization in the MR element, and a unidirectional longitudinal field component for maintaining a domain-wall-free magnetization state in the active region of the MR element. Since the PM element 12 is located away from the head-medium interface, this geometry has the advantage that the PM element 12 is encapsulated in the integrated head and is therefore not exposed to corrosion at the head-medium interface 16. It has the further advantage that the PM element 12 is located a distance from the head-medium interface 16 and is therefore unlikely to distort the information recorded on the magnetic medium. However, the geometry disclosed in the '284 patent has the drawback that the longitudinal component of the magnetic field produced by the PM element 12 is more uniform along the length of the MR element than is desirable for optimum reproduce sensitivity.
A need has therefore been felt for an improved MR head of the type having a permanent magnet for biasing an MR element in the head that avoids uniformity and provides much larger gradients along the length of the MR element and provides the large bias fields at the ends of the MR element to help stabilize the single domain magnetization state.